Physical vapor deposition (PVD) is a useful technique for producing thin films, enabling the fabrication of devices such as organic light emitting diodes (OLEDs) and photovoltaic devices (“solar cells”). Shadow masks are used to pattern the thin films during the deposition process. In general, there is a need for precision alignment between the various layers deposited in the fabrication of complex, layered structures such as OLEDs.
Existing methods for aligning a shadow mask to a substrate are generally divided into two categories, active and passive. In an active approach a sensing system, generally a computer vision system, is used to determine the relative position error between the shadow mask and the substrate. Visible fiducials are provided on the shadow mask and the substrate, and then the position error is measured by computing the distance and direction between a pair of corresponding fiducials. A precision motion system is then employed to reposition the mask or the substrate in order to reduce or remove the error. Active alignment systems tend to be fairly expensive due to the cost of the computer vision system, and can be somewhat slow due to the requirements for highly precise motion mechanisms to correct errors. U.S. Pat. No. 6,729,927 describes the use of computer vision for aligning shadow masks to a frame.
Passive alignment generally relies on kinematic constraint to produce the desired relative positioning. U.S. Pat. No. 4,915,057 is typical of this approach. In this passive system a frame is used to hold the shadow mask. A set of alignment holes in the shadow mask frame is mated to corresponding pins in an alignment frame. The substrate is likewise mounted in a frame with alignment holes, which are then mated to a different set of pins in the alignment plate. (The method of U.S. Pat. No. 4,915,057 actually requires additional alignment steps in which the mask is aligned to the mask frame. Likewise, the substrate should be aligned to the substrate frame.) This passive alignment process can be faster than active alignment, but suffers from what is known as tolerance stack-up. Since there are no perfect alignment features and holes, and no perfect placements, each stage of contact or offset in passive alignment introduces a new source of tolerance stack-up. Although these effects can be reduced to very low levels, this requires expensive fabrication techniques.